1. Technical Field
The present invention relates to a receiving device that receives a satellite signal from a positioning satellite, and the like.
2. Related Art
A global positioning system (GPS) is widely known as a positioning system using a satellite signal and is used for a receiving device built in a mobile phone, a car navigation apparatus, and the like. In the GPS, a position calculating operation of calculating the position coordinate and the clock error of a receiving device is performed on the basis of positions of plural GPS satellites, pseudo-distances from plural GPS satellites to the receiving device, and the like.
In environments in which a received GPS satellite signal has a weak electric field state (weak electric field environments), a received signal is weak, a correlation value obtained through a correlation operation is small, and it may be thus difficult to detect a peak of the correlation value. Therefore, particularly in weak electric field environments, a technique of integrating correlation values, which have been obtained by performing a correlation operation, over a predetermined period of time is used (for example, see US-A-2006-0133463).
When a GPS satellite signal is actually received (when capturing a signal), a receiving frequency includes an error due to a Doppler frequency. That is, a Doppler shift occurs depending on a variation in relative positional relationship between a GPS satellite and a receiving device and the receiving frequency is shifted from 1.57542 [GHz] which is a carrier frequency of a GPS satellite signal.
The Doppler shift has a great influence on a correlation integrating process of integrating correlation values. This will be specifically described. A correlation value for each phase is obtained through a correlation operation on a received signal and a replica code. A code phase is specified on the basis of a phase (hereinafter, referred to as a “peak phase”) corresponding to the maximum correlation value and a pseudo-distance is calculated on the basis of the specified code phase and is used for positioning.
However, the peak phase is shifted with the lapse of time due to the influence of the Doppler shift. Therefore, when a correlation value for each phase obtained through the correlation operation at a certain time and a correlation value for each phase obtained at a subsequent time are sequentially and repeatedly integrated for each corresponding phase, the peak phase varies and is superposed and thus the shape of the correlation value (hereinafter, referred to as an “integrated correlation value”) integrated for each phase is blunted as a whole, thereby making it difficult to distinguish the peak phase. Therefore, a technique of integrating the correlation values with a phase shift when integrating the calculated correlation values for each phase is generally used in the related art. This correlation integrating process is the same in positioning systems other than the GPS.